复合材料非对称正交薄层板的固化变形

利用Rayleigh-Ritz 法研究了复合材料非对称正交薄层板的固化变形。建立了考虑几何非线性的固化变形分析模型, 预报了其固化后的变形形状及变形量。利用热压釜工艺进行了实验研究。实验发现, 方板边缘发生了较大的向内卷曲变形, 板边缘附近理论预报值与实验结果差别较大, 在距板边缘一定距离内理论预报值和实验结果吻合较好, 矩形板实验结果与理论预报值吻合良好。      

双稳定矩形非对称复合材料层板的跳变研究

对双稳定矩形非对称复合材料层合板两种稳定构型的跳变行为进行了实验和数值研究。通过实验测得中心加载下层合板两种稳定构型跳变的临界载荷, 建立了两种稳定构型的非线性有限元分析模型, 成功预报了层合板固化后的两种稳定构型, 并进一步预报了两种稳定构型的跳变临界载荷, 预报结果与实验结果吻合良好。实验和有限元模拟结果表明, 中心加载下矩形非对称层合板两种构型的临界载荷相差较大, 并且跳变的过程有所差异。      

Environmental effects on thermally induced multistability in unsymmetric composite laminates

The principles involved in the generation of thermal induced multistability in carbon fibre epoxy laminates have received much interest in the published literature. This work examines the effects of moisture absorption on the mechanical properties of these plates focussing on geometry and ‘snap-through’ loadings. Samples were monitored from a dry state until moisture equilibrium was achieved. It was observed that substantial changes in geometry and snap-through performance occurred as moisture content increased. As part of this work, a first order strain energy analysis was modified to incorporate a hygrothermal strain term to enable prediction of the laminate shape due to moisture content.     

Characterisation and modelling of the cured shapes of arbitrary layup bistable composite laminates

Asymmetric laminates can have a bistable response to loading, making them of particular interest for applications requiring a large deflection with relatively small and removable energy input. After nearly 30 years of research effort the effects of ply orientation and laminate geometry on the room-temperature shapes of such laminates are well understood and the temperature dependent deformations of laminates of arbitrary layup can be quantitatively predicted. With attention switching to the design of mechanisms incorporating bistable laminates for actuation there is an increasing importance placed on the accuracy and the sensitivity of modelling techniques to predict out-of-plane deflections. This paper presents the results of a novel experimental investigation to map the surface profiles of a series of arbitrary layup laminates. It is concluded that while existing modelling techniques are successful in accurately predicting room-temperature shapes, the sensitivity of solutions to imperfections is significant.     

固化度与固化收缩对非对称复合材料层合板固化变形的影响

采用三维有限元方法研究复合材料非对称层合板在热载荷和固化收缩载荷下的固化变形情况, 建立了材料力学特性、 固化体积收缩量和温度与固化度之间的函数关系, 考察了层合板变形曲率与温度和固化度之间的关系。数值计算结果表明: 非对称层合板变形曲率与固化终止时固化度有密切关系; 固化变形主要发生在降温阶段; 固化收缩对层合板变形曲率影响很小, 主要发生在第二个保温平台的前半段。      

Nondestructive evaluation of thermal stress-induced damage in thin composite laminates

Nondestructive evaluation of thermal stress-induced failure in the free edge region of thin composite laminates (1 mm thick) has been carried out. The damage zone which arose under a cryogenic cooling was examined using the ultrasonic C-scan and optical microscopy. Characteristic behavior of thermo-acoustic emission (AE) detected from the composite specimen during the heating and cooling cycle indicated the degree of damage which had occurred differently for as-moulded and cryogenically-treated specimens with various lay-up angles. Analysis of the vibration spectrum data obtained with short beam specimens exhibited that the failure occurrence caused an obvious decrease in resonant frequency and some considerable increase in flexural damping ratio. Total AE energy showed a feature similar to the flexural damping ratio which increased with increasing crack density. Consequently, it is thought that thermo-acoustic emission in association with flexural damping might be employed as a tool for nondestructive evaluation of thin composite laminates.     

Phenomena in the bifurcation of unsymmetric composite plates

The phenomena observed in the large scale bifurcation of unsymmetric composite plates are characterized. Such plates can exhibit more than one stable geometry and can be ‘snapped’ from one geometry to another. It is convenient to think of this process as providing sufficient strain energy to overcome an energy maximum and is analogous to an activation energy. To describe the snap-through process, 2-ply, [0, 90] unsymmetric plates were forced from one state to the other in rigid test rigs such that great control could be achieved over the process and loads and deformations were monitored throughout the bifurcation. A hitherto unrecognised phenomenon was noted in that the snap-through process was seen to take place not as a single bifurcation but as two closely coupled bifurcations, as first one side snaps through and then the other. The experimental approach taken here, coupled with the simple energy analysis, should be of value in understanding practical structures of multiple stable states, such as might be used in morphing or adaptive aircraft.     

Experimental study of the impact behavior of repaired thin laminates with double composite patch

Abstract

The aim of this article is to present a study of the behavior of patch-repaired laminates subjected to low-velocity impacts. A broad range of impact energies was selected. Results for repaired laminates in terms of contact load, damage and absorbed energy were compared to those obtained from intact specimens. At impact energies below 10?J, energy absorption in repaired specimens was higher than the one given in intact laminates, although the measured damage area was found to be greater in the former configuration. For higher impact energies, both damage area and energy absorption in intact specimens were greater than in repaired laminates.     

Comparison of different setups for fatigue testing of thin composite laminates in bending

After developing a damage model, validation of this model is a necessity. This validation should preferably be done under loading conditions which are different from the ones used for the development of the model.This study investigates whether a special design of a bending setup is suited for the mechanical testing of thin fibre-reinforced composites with a low bending stiffness and the validation of fatigue material models, developed in uni-axial loading conditions. First, the disadvantages of a three-point bending setup for thin laminates are commented on. Then, a four-point bending setup is discussed, for which interestingly enough, the finite-element model can be simplified. Clamped supports are added to decrease the midspan displacement. This is followed by the discussion of the clamped three-point bending setup.It may be concluded that the clamped three-point bending setup is very promising for experimental work, since it increases the measured loads and decreases the midspan displacement with respect to the regular three-point bending setup, although further modification of the setup is necessary to avoid slipping in the grips.The material used for this study was a carbon fabric-reinforced polyphenylene sulphide.     

Biodegradability of all-cellulose composite laminates

The high mechanical properties of single-polymer composites based on degradable non-derivatised cellulose, aka all-cellulose composites, have recently captured the attention of researchers. All-cellulose composites possess the intriguing combination of high strength and biodegradability. However, the biodegradation behaviour of all-cellulose composites has so far not been reported. In this work, soil burial experiments were carried out to compare the biodegradation behaviour of all-cellulose composites with conventional biocomposites in order to investigate the end-of-life disposal of this relatively new class of bio-based composite materials. All-cellulose composites are characterised by exceptional biodegradability with mass losses of up to 73% following a soil burial time of 70 days. An investigation of the mechanisms of biodegradation of all-cellulose composites is undertaken for the first time.     

正交铺设碳纤维复合材料结构的双稳态特性研究

对不同几何尺寸的[0°/90°]正交非对称铺设碳纤维复合材料的双稳态特性进行了研究。在分析正交非对称复合材料双稳态特性产生机理和理论基础上,成功制备了碳纤维/环氧树脂(T700/3234)正交铺设的双稳态试样。对制备固化冷却后产生的圆柱状第一稳态、建立实验平台实现向第二稳态转变过程,经过逆向加载返回至第一稳态的整体稳态间转变总过程进行实验研究,分析稳态曲率半径、面外位移、稳态间转变最大载荷及载荷-位移曲线等的变化规律。     

Macromechanical behaviour of composite laminates

An improved higher order transverse shear deformation theory is employed to arrive at modified constitutive relations which can be used in the flexural, buckling and vibration analysis of laminated plates and shells. The strain energy for such systems is then expressed in terms of the displacements and the rotations for ready reference and use. Numerical values of vibration frequencies are obtained using this formulation employing Ritz's method of analysis. The results are compared with those available in the literature to validate the analysis presented.     

Review of z-pinned composite laminates

This paper reviews published research into polymer composite laminates reinforced in the through-thickness direction with z-pins. Research into the manufacture, microstructure, delamination resistance, damage tolerance, joint strength and mechanical properties of z-pinned composites is described. Benefits of reinforcing composites with z-pins are assessed, including improvements to the delamination toughness, impact damage resistance, post-impact damage tolerance and through-thickness properties. Improvements to the failure strength of bonded and bearing joints due to z-pinning are also examined. The paper also reviews research into the adverse effects of z-pins on the in-plane mechanical properties, which includes reduced elastic modulus, strength and fatigue performance. Mechanisms responsible for the reduction to the in-plane properties are discussed, and techniques to minimise the adverse effect of z-pins are described. The benefits and drawbacks of z-pinning on the interlaminar toughness, damage tolerance and in-plane mechanical properties are compared against other common types of through-thickness reinforcement for composites, such as 3D weaving and stitching. Gaps in our understanding and unresolved research problems with z-pinned composites are identified to provide a road map for future research into these materials.     

Fire behaviour of composite laminates

The thermal response of laminated glass fibre reinforced panels to severe fire conditions has been investigated by furnace fire testing and thermal modelling. Excellent fire resistance has been demonstrated for several matrix materials and the materials response has been modelled to a high degree of accuracy. The thermal resistance properties are due to a combination of low thermal conductivity, good structural integrity and significantly, the endothermic decomposition of the matrix, which slows down the heat transmission through the laminate.     

Fracture of fatigue-loaded composite laminates

While the quasi-static fracture load of many composite laminates can be estimated with engineering accuracy, the fracture event itself has not been clearly characterized and is incompletely understood. When cyclic loading is present, the pre-fracture damage state is altered significantly, so that estimating strength (or residual strength) is greatly complicated. The present paper examines this complexity and attempts to assess the manner in which pre-fracture fatigue damage affects residual strength and the fracture event. It is found that the large strength reductions observed prior to failure at low load levels can be accounted for by internal stress redistribution and material degradation events. A careful chain of physical evidence in support of this approach is presented.     

Failure analysis of low velocity impact on thin composite laminates: Experimental and numerical approaches

The dynamic behavior of composite laminates is very complex because there are many concurrent phenomena during composite laminate failure under impact load. Fiber breakage, delaminations, matrix cracking, plastic deformations due to contact and large displacements are some effects which should be considered when a structure made from composite material is impacted by a foreign object. Thus, an investigation of the low velocity impact on laminated composite thin disks of epoxy resin reinforced by carbon fiber is presented. The influence of stacking sequence and energy impact was investigated using load–time histories, displacement–time histories and energy–time histories as well as images from NDE. Indentation tests results were compared to dynamic results, verifying the inertia effects when thin composite laminate was impacted by foreign object with low velocity. Finite element analysis (FEA) was developed, using Hill’s model and material models implemented by UMAT (User Material Subroutine) into software ABAQUS™, in order to simulate the failure mechanisms under indentation tests.     

Analysis of unsymmetric CFRP–metal hybrid laminates for use in adaptive structures

The potential of using multistable composite materials for adaptive structures is currently receiving interest from the aerospace community because they possess more than one single equilibrium configuration. Unsymmetric CFRP laminates are studied which have an inner isotropic metallic layer. These hybrid laminates are studied using analytical, finite element and experimental techniques. The thermal contraction of the isotropic layer upon cool down from cure induces large in-plane thermal loads which act remotely from the laminate’s neutral plane, increasing snap-through moments and out-of-plane displacements. The curvatures of the hybrid laminates can be doubled compared to pure unsymmetric CFRP laminates.     

Dynamic characteristics of thermoplastic composite laminates

Dynamic characteristics of carbon fibre/polyetheretherketone (cf/peek) composites have been investigated by the impact test and the sinusoidal free vibration test. Using a cantilevered beam with rectangular cross-section, the natural frequencies and damping properties were measured. Also, dynamic tests for the cantilevered beam with a torsional bar have been performed to identify the shear modulus and damping in torsional motion. Results for the specific damping capacity given by the impact test are very close to those given by the sinusoidal vibration test. The elastic moduli from the impact test are slightly smaller than those from the tension test. The dynamic characteristics of angle-ply, cross-ply and quasi-isotropic laminates have been computed by the finite element method. For the purpose of verification, experimental tests were carried out by the impact test for these laminates. The experimental values are very close to the numerical values. In addition to cf/peek composites, experiments have been performed for cf/epoxy composites. The damping of cf/epoxy is much larger than that of cf/peek.